Investigations into the anchoring of liquid crystals (LCs) on self-assembled monolayers (SAMS) formed from .omega.-functionalized alkanethiols supported on films of gold deposited obliquely onto glass substrates has recently been reported (Gupta et al., Langmuir 10: 12, 2587 (1996); Gupta et al., Chem. Mater. 8: 1366 (1996)). This system is a useful one because, with relative ease, it is possible to design and synthesize surfaces that differ in structure and presentation of functional groups, and thus explore the influence of the structure of surfaces on the orientations of supported LCs (Gupta et al., Langmuir 10: 12, 2587 (1996); Gupta et al., Chem. Mater. 8: 1366 (1996); Gupta et al., Science 279: 2077 (1998)). For example, by hosting recognition moieties that recognize particular analytes within SAMs, it is possible to amplify and transduce the interaction between the recognition moiety and the analyte on these surfaces into optical signals through the orientations of birefringent LCs placed on these surfaces See, U.S. patent application Ser. No. 09/127,383, filed Jul. 31, 1998 entitled "Optical Amplification of Molecular Interaction Using Liquid Crystals." This approach makes possible detection of the interactions of analytes with surfaces (e.g., antibody-antigen interactions) with relatively simple procedures involving LCs and visual inspection.
Past studies of SAMs on metals have focused on the use of single crystals of metal (Strong et al., Langmuir 4: 546 (1988); Widrig et al., J. Am. Chem. Soc. 113: 2805 (1991)), metals deposited by electroless methods (Hou, et al., Langmuir 14: 3287 (1998), or evaporated films of metal deposited without a preferred direction (Gupta et al., Chem. Mater. 8: 1366 (1996); Drawhom et al., J. Phys. Chem. 45: 99, 16511 (1995)) (FIG. 1A).
Recent work dealing with LCs has relied on the use of gold films deposited at an oblique angle of incidence with respect to a substrate of glass (Gupta et al., Langmuir 10: 12, 2587 (1996); Gupta et al., Chem. Mater. 8: 1366 (1996) (FIG. 1B). LCs supported on SAMs formed on obliquely deposited gold (100 .ANG. of gold deposited at an angle of 50.degree. from the normal) generally have been observed to assume one of two azimuthal orientations (defined with respect to the plane of incidence of the gold during deposition of the gold film) (Gupta et al., Langmuir 10: 12, 2587 (1996)). For example, nematic phases of 4-cyano-4'-pentylbiphenyl (5CB) supported on SAMs formed from CH.sub.3 (CH.sub.2).sub.15 SH assume an orientation that is parallel to the surface of the substrate and parallel to the plane of incidence of the gold during deposition of the film of gold. In contrast, nematic phases of 5CB supported on SAMs formed from CH.sub.3 (CH.sub.2).sub.14 SH assume a bulk orientation that is parallel to the surface of the gold substrate but perpendicular to the plane of incidence of the flux of gold. Certain nanometer-scale structures formed on these surfaces (e.g., protein molecules bound specifically to ligands hosted on these surfaces) can erase the influence of the oblique deposition of the gold on the preferred azimuthal orientations of a supported LC. This fact is exploited in past work dealing with the development of principles for amplification and transduction of ligand-receptor interactions at surfaces (Gupta et al., Science 279: 2077 (1998)).
A number of past studies of obliquely deposited metals and metal oxides have been reported. The films investigated in those studies were thick (&gt;50 nm) and prepared by sputtering, giving rise to large grains, columnar structures, and anisotropy that was easily seen by visual inspection of, for example, SEM or TEM images of cross sections of the films. (Jerome, B., Rep. Prog Phys., 54: 391 (1991); Tait et al., J. Vac. Tech. A. 4: 10, 1518 (1992); van Kranenburg, et al., Mat. Sci. & Engr., 7: R11, 295 (1994); Vernier et al., J. Vac. Sci. Tech. A 3: 9, 563 (1991); Mbise et al., Thin Solid Films 174: L123 (1989); Goodman et al., IEEE Trans. Elec. Dev., 7: ED-24, 795 (1977)). In contrast, thin films of obliquely deposited gold (.about.30 nm, so as to be semi-transparent), are formed of small grains with characteristic dimensions of &lt;30 nm, and have no obvious anisotropy when images (real or Fourier space) obtained by AFM or STM are inspected visually.
Although studies have demonstrated that obliquely deposited gold films are useful substrates for SAMs when anchoring LCs, the structure of these gold films has not been reported in detail. It is noteworthy that preliminary attempts to characterize the anisotropy within these gold films by visual inspection of scanning tunneling micrographs were largely unsuccessful. See, Gupta et al., Langmuir 12: 2587 (1996).
Surprisingly, the present invention provides experimental procedures and methods of analysis that provide quantitative measures of the anisotropy within obliquely deposited metal films. By the use of scanning probe methods (AFM/STM), it is possible to routinely and reproducibly detect anisotropy within thin, obliquely deposited metal films and organic layers attached to these films. Further, the anisotropy measured can provide an account of the preferred azimuthal orientation of LCs on obliquely deposited metal films.